Thursday, January 15, 2015

Want to keep it hot? The answer is Leather.

By Natalie Claunch

Quick, think of a turtle. Does it have a shell? Now, imagine
it in its habitat. Where is it living? Did you pick a tortoise in a hot desert?
A pond turtle basking on a sunny log? A sea turtle cruising in the Caribbean?

Was your turtle living in a warm place? If so, your
intuition was good. Most reptiles, like turtles, regulate their temperature by
basking in warm environments, and moving elsewhere when they get too hot. In
other words, reptiles thermoregulate behaviorally because they are not capable
of producing body heat to keep warm, and must move to shade to keep cool.

Speaking of cool, I’d like to introduce you to the one of
the coolest reptiles around: Leatherback Sea Turtles, Dermochelys coriacea. In fact, they keep themselves from becoming
giant ice cubes while swimming in frigid water as low as 0.4 degrees Celsius.(James, Davenport & Hayes, 2006).

They are the only living members in the family Dermochelyidae,
but they come from a long line of cold-adapted sea turtles… Fossils of their ancestors
have been found in Antarctica, and estimated seawater temperatures for these
extinct relatives were still as low as 6 degrees C (Albright, et al 2003).

These guys also have the largest migratory path of any
reptile, feeding in temperate waters and breeding in tropical waters for years
on end. Here we’ll focus on the amazing suite of physiological and behavioral
adaptations that allow a reptile to take advantage of frigid habitat.

Just how does a turtle keep warm when it must forage in cold
environments? Well first off, they’ve got size on their side.

Just look at the size of this fella!

Leatherbacks are the largest turtles in the world. Their
huge size contributes to heat retention in cold environments. By having a low
surface area- to volume ratio, the turtles dissipate heat much slower than a
smaller counterpart. Leatherbacks also have a fatty insulation layer, similar to
that found in marine mammals. However, they do not have this insulation in
their fins, which make up as much as 30% of their exposed surface area. Scientists
have found that heat loss in flippers is much less than expected in these
environments (Bostrom et. al 2010), suggesting a reduction in bloodflow to the
extremeties or a countercurrent heat exchanger mechanism, much like that described
in aquatic birds. Penick et al 1998 found that the pectoral muscles in
leatherbacks had thermally independent metabolism, allowing the pectoral muscle
to function normally in temperatures from 5 C to 25C. This was not the case in
Green Sea Turtles, whose pectoral muscles decreased in function as temperature
decreased.

Warm blood stays near the body core, so heat is conserved

This combination of physiological adaptations has been
termed gigantothermy. And as awesome as the term sounds, it just doesn’t seem
to explain how leatherbacks can manage to keep warm. Gigantothermy assumes that
leatherbacks have low metabolic rates, just like other reptiles.If so, how can these turtles really combat
heat loss? After all, these turtles live
in cold water for months at a time, so are constantly losing heat. These
turtles aren’t just losing heat through the outside of their bodies. They are
also losing heat at the very core their giant bodies are supposed to keep warm—every
time they ingest their prey! The jellyfish they hunt are about the same
temperature as the water they are diving in.

Leatherback eating a jelly at the surface

By eating cold prey, leatherbacks spend energy (heat) to
warm the prey to body temperature. In fact, it is a crucial survival tip to NOT eat snow when thirsty and stranded in arctic environments, because more energy
is spent warming the snow to water than gained by the resulting gulp! Similar
to a stranded hiker melting snow over a fire before drinking, there are a few
accounts of turtles seemingly combating this heat differential by carrying prey
to the surface to ingest. This would allow the prey to warm at least to the
temperature of the surface—but this does not seem to be a main strategy.

Don't touch them. They're cold.

A recent study found
that Leatherbacks keep their body temperatures within a 3 degree temperature
range, and at times this is up to ten degrees warmer than water temperatures
(Casey, James, & Williard 2014). Modelling suggests that these turtles must have higher metabolic rates than other reptiles to maintain a
body temperature that high while ingesting cold prey. Casey, James and Williard 2014 proposes that
leatherbacks use the heat produced from specific dynamic action to warm prey.
Specific dynamic action (SDA) refers to energy above resting metabolic rate
that is required to break down and store food. A byproduct of these energy
processes is heat- and it is estimated up to 90% of the heat produced by
SDA in leatherbacks goes to warming prey to body temperature. Leatherbacks also possess brown fat- which could aid in keeping warm as well. For more info on brown fat, check out this recent post. This is really
cool, because it means these reptiles are able to produce metabolic body heat—something
a true ectotherm cannot rely on to survive.

Apart from physiological mechanisms, Leatherbacks also thermoregulate
behaviorally. They don’t bask like most reptiles, however. They can adjust their
dives to become shallower and avoid the coldest parts of the deep. They can
also change their swimming speed to keep warm. Turtles have even been shown to
exercise to keep warm, by increasing paddling rate in colder water.

So, to sum up: Leatherbacks have lots of adaptations to
prevent heat loss, retain and even produce heat, and adjust their body
temperature behaviorally. They are truly unique among reptiles, and deserve
more research attention.

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